The invention relates to a process for the production of a diene that comprises a stage a) for decomposition of at least one tertiary alkyl ether into at least one tertiary olefin of high purity, a stage b) for purification of the tertiary olefin and a stage c) for oxidizing dehydrogenation of this tertiary olefin into a diene.
It relates in particular to a process for the production of isoprene from the decomposition of tert-amyl-methyl-ether (TAME) or ethyl-tert-amyl-ether (ETAE) into isoamylenes of high purity that contain 2-methyl-but-1-ene and 2-methyl-but-2-ene, the separation of isoamylenes from methanol or respectively ethanol, and the oxidizing dehydrogenation of these isoamylenes into isoprene (2-methyl-buta-1,3-diene).
The first stage of the process according to the invention is a method for the production of high-purity isoamylenes from an ether, TAME or ETAE whose production makes it possible, for example, to upgrade an olefinic fraction with five carbon atoms that contains at least one 2-methylbutene, such as the one that is produced by the catalytic cracking on the moving bed (called F.C.C, whose initials come from the English xe2x80x9cFluid Catalytic Crackingxe2x80x9d), steam cracking, the dehydrogenation of isopentane or isomerization of olefins with five carbon atoms. Their synthesis results from the selective addition of methanol, or ethanol on isoamylenes (2-methyl-but-1-ene and 2-methyl-but-2-ene). It involves balanced reactions that can be enhanced within the framework of synthesis of TAME and ETAE as within the framework of their decomposition.
In the latter case, it is possible to obtain, in a selective manner, the isoamylenes with a high purity. This process then makes it possible to prevent the distillation of the isoamylenes of a fraction with five carbon atoms, a difficult process taking into account small differences between the boiling points of the different olefins with five carbon atoms.
By contrast, it is generally easy to separate the TAME or the ETAE from the hydrocarbon fraction from which it is obtained. Once isolated, the ether can be decomposed again to form the initial tertiary olefin and the alcohol that is used. This takes place by an endothermic process in the presence of a generally acid catalyst and at a higher temperature than for the synthesis. The tertiary olefin that is produced can then have high purity based on operating conditions. In a second stage of the process, the tertiary olefin is dehydrogenated to form a diene, preferably isoprene.
The polymerization of the isoprene results in cis-1,4-polyisoprene, an equivalent of natural rubber, which exhibits excellent purity and a homogeneity that is greater than that of natural rubber.
It has been shown that isoprene can be produced from a process of three successive stages. U.S. Pat. No. 3,391,214 describes the production of isoprene from isopentane that undergoes a scheme of three reactions (a hydroperoxidation and two epoxidations) in the presence of catalysts that are specific to each stage of the process. The feedstock that is used, however, is isopentane.
The process for decomposition of the tertiary alkyl ethers into tertiary olefins has been known for a long time, as, for example, Patent Application EP-A-0 068 785 shows, and various acidic solids have been described as catalysts of these reactions. Patent Application FR-A-2 291 958 relates to a process for decomposition of TAME or ETAE respectively into isoamylenes and methanol or isoamylenes and ethanol, with use of catalysts that are selected from among the salts, oxides or complexes of tetravalent uranium and can be supported on an alumina-a, for example, that has a Lewis acidity.
International Application WO-A-91/01 804 describes the production of isoamylenes from TAME with a clay catalyst that is treated with an acid that is selected from among hydrofluoric acid, hydrochloric acid and a mixture of hydrofluoric and hydrochloric acids.
U.S. Pat. No. 5,227,564 describes the decomposition of TAME in a vapor phase and in the presence of a catalyst that contains a silica-alumina zeolite, and Patent Application EP-A-0 589 557 and U.S. Pat. No. 4,536,605 describe the use of a catalyst with a calcined silica-alumina base. U.S. Pat. No. 5,171,920 describes the process for obtaining at least one tertiary olefin by decomposition of the corresponding ether, either TAME or ETAE, with a catalyst that consists of silica that is modified by the addition of at least one element, such as Li, Cs, Mg, Ca or La, for example. Such solids are not very active due to the absence of acidity, and they have a mediocre stability over time: the data of Table 1 of Example 13 of said patent indicate that in 800 hours, it is necessary to increase the temperature of 50xc2x0 C. to keep the ether conversion level constant.
These catalysts that are based on alumina, silica or silica-alumina require the addition of water to improve the recovery of the alcohol and to prevent the secondary reaction of formation of the corresponding dialkyl ether, which is, for example, dimethyl ether (or DME) in the case of the methanol:
2MeOHMexe2x80x94Oxe2x80x94Me+H2O 
This is described in particular in Patent Applications GB-A-1 165 479 and EP-A-0 589 557. The presence of water, however, lowers the activity of the catalyst by lowering its acidity (see in particular Patent Application GB-A-1 165 479) and can then make it necessary to operate at a higher temperature, which can interfere with the service life of the catalyst. In addition, the presence of water induces an additional secondary reaction: the water reacts with the tertiary olefins to form an alcohol, such as, for example, in the case of isoamylene to form 2-methyl-butan-2-ol. According to this process, therefore, a loss of the tertiary olefin yield is recorded.
Finally, U.S. Pat. No. 5,095,164 describes a process for decomposition of tertiary alkyl ethers such as TAME or ETAE that use ion exchange resins, for example the sulfonated styrene-divinylbenzene resins. It is thus possible to cite the resin Amberlyst 15(R) of RHOM and HAAS or the resin M-31(R) that is marketed by DOW CHEMICAL. U.S. Pat. No. 4,447,668 also uses an ion exchange resin for producing isoamylenes and diisoamylenes from the separation of TAME.
One of the major drawbacks of the resins that are cited above is the impossibility of using them at high temperature, more specifically above 120xc2x0 C. Actually, at high temperature, these resins lose sulfonic groups and therefore lose at least in part their activity and/or their acidity. The decomposition reactions of the ethers are endothermic, however; the thermodynamic equilibrium of the reaction is therefore shifted toward olefin production the higher the temperature. An operating temperature that is limited to 120xc2x0 C. is reflected by weak ether conversion and limited by the laws of thermodynamics.
U.S. Pat. No. 5,095,164 that is cited above uses distillation equipment with a catalyst that is placed at the bottom of a column and that operates between 50 and 100xc2x0 C., preferably between 60 and 80xc2x0 C. The thermodynamic equilibrium of the decomposition reaction, which is poorly placed due to the fairly low operating temperature, is shifted by the elimination of the reaction products (tertiary olefin and corresponding alcohol) by distillation. Such a process, however, presents difficulties for the purification of the products. In particular, it uses large amounts of water for the extraction and/or the recovery of the alcohol. In addition, the unconverted ether is recovered at the bottom of the column with significant amounts of alcohol. It should then be purified before being recycled in the process.
The oxidative dehydrogenation of the olefins to form dienes is a process that has been known for a very long time by ones skilled in the art. It is carried out in the presence of catalysts of any type, but that usually contain iron, oxygen and another metallic element. The reaction for oxidizing dehydrogenation of isoamylenes into isoprene is described in, for example, the Encyclopedie Ullmann de Chimie Technique [Ullmann""s Encyclopedia of Technical Chemistry], 4th Edition, Volume 13, pages 381-382. Various reaction variants of oxidative dehydrogenation have been proposed.
U.S. Pat. No. 3,156,735 describes the oxidative dehydrogenation of isoamylenes into isoprene, in the presence of a catalyst that contains gold in combination with a noble metal that is selected from the group that consists of platinum, palladium, rhodium, ruthenium and iridium, deposited on a solid refractory oxide substrate that has a small specific surface area.
More recently, U.S. Pat. No. 4,973,793 sets forth the use of a catalyst that is based on iron, oxygen and zinc, in the presence of oxygen and vapor, in the oxidative dehydrogenation reaction of a feedstock that contains isoamylenes and butenes for the production of isoprene. The presence of butenes makes it possible to increase the conversion level of the isoamylenes. This yield decreases, however, if the butene concentration is greater than 80% moles.
This invention relates to a process for the production of a diene that comprises a decomposition stage a) of at least one tertiary alkyl ether in a mixture that contains at least one tertiary olefin, at least one alcohol and at least one residual ether, in the presence of a catalyst that comprises at least one mineral solid that is grafted by at least one organic group such as alkyl sulfonic acid, aryl sulfonic acid or alkylaryl sulfonic acid, a stage b) for purification of the tertiary olefin that is obtained in stage a) and a stage c) for oxidizing dehydrogenation of the tertiary olefin that is obtained in stage b), in the presence of a catalyst under conditions of obtaining a diene.
In the first stage of the process according to the invention, the tertiary alkyl ether is decomposed into a tertiary olefin of high purity, in the presence of a catalyst that comprises at least one mineral solid, for example of polysiloxane type, grafted by at least one organic group such as alkyl sulfonic acid that usually contains from 1 to 24 carbon atoms, preferably 2 to 20 and even more preferably from 2 to 5 carbon atoms, aryl sulfonic acid that usually contains from 6 to 18 carbon atoms, or alkylaryl sulfonic acid that usually contains from 7 to 24 carbon atoms. More particularly, the tertiary alkyl ether is TAME or ETAE that decomposes into isoamylenes that are 2-methyl but-1-ene and 2-methyl but-2-ene (whereby the 3-methyl but-1-ene is not very reactive) and into the corresponding alcohols (methanol or ethanol).
Said catalyst that preferably comprises a solid such as polysiloxane that is grafted by at least one organic group is in particular marketed under the registered trademark xe2x80x9cDELOXANxe2x80x9d, which contains a polymeric unit of the formula: 
The preparation of such a solid is described in, for example, Patents U.S. Pat. No. 4,552,700, U.S. Pat. No. 5,354,831 and U.S. Pat. No. 5,380,791. These solids have a strong Brxc3x6nsted acidity. The possibility of using such solids in ether decomposition reactions is mentioned in, for example, Patent FR-B-2 747 120.
These catalysts are very active for the decomposition reaction of tertiary alkyl ethers into the corresponding tertiary olefins. They make it possible to operate at a low temperature, for example at 130xc2x0 C., which promotes good stability of the catalyst and can also be reflected by a very long service life of the catalyst. If the endothermicity of the reaction is also taken into account, and therefore the favorable shifting of the thermodynamic equilibrium by a temperature elevation, it is possible to work, with these catalysts, at high temperature (typically, for example, above 150xc2x0 C. and often between 180xc2x0 C. and 220xc2x0 C.)
The fact that the grafted solid is mineral and non-organic, for example such as polysiloxane, makes it possible to work at such temperatures without noteworthy degradation of the catalyst. In the case of use of high temperatures in the first stage of the process according to the invention, the strong activity of the catalysts that are used according to the process of this invention makes it possible to work with high L.H.S.V. (hourly liquid volumetric flow rate, expressed in volume of liquid feedstock per volume of catalyst and per hour). This is reflected by the reduction of the necessary catalytic volume and also by the reduction of installation equipment (hence a double economic advantage). In addition, the possibility of working with high traverse speeds (high L.H.S.V.) reduces the part of secondary reactions; very good yields of tertiary olefins and alcohol are ultimately obtained.
The catalyst that is used in stage a) of this invention has excellent stability over time for the decomposition reaction, which facilitates the operations and offers a certain economic advantage, based on improvements such as less frequent rest periods and an overall savings on the cost of the catalysts.
The operating conditions for the decomposition stage of the tertiary alkyl ether are as follows. The (relative) pressure is generally between 0.1 to 3 MPa, preferably between 0.1 and 1 MPa. The temperature is from 100 to 230xc2x0 C., preferably between 120 and 200xc2x0 C., and the hourly volumetric flow rate per volume of catalyst per hour (L.H.S.V.) is from 1 to 200 hxe2x88x921, often between 1 to 100 hxe2x88x921. Thus, it is preferred to work between 120 and 200xc2x0 C. with a L.H.S.V. of about 2 to 50 and preferably about 2 to 20. The selection of the pair of parameters (temperature, L.H.S.V.) is essential for optimizing the performance levels of the catalyst (optimum conversion of the TAME and good selectivities of alcohol and isoamylenes, i.e., considerably limited secondary reactions).
Stage b) of the process according to the invention is a purification of the tertiary olefin that is obtained in stage a) for extracting said olefin from the mixture that is obtained from stage a). The purification comprises at least one stage, most often two stages. By way of example, the purification can comprise at least one water washing stage and at least one distillation stage, whereby the washing and the distillation are carried out in any order. The washing is preferably carried out before the distillation.
The operating conditions of the washing with water of stage b) for purification of the olefin comprise an amount of water such that the volumetric ratio between the water volume and the volume of the mixture that is obtained in stage a) is usually from about 0.005 to 100, most often from about 0.01 to 20, preferably from about 0.1 to 10 and even more preferably from about 0.5 to 5. This washing most often uses a plate column that operates at a temperature from about 1 to 100xc2x0 C., preferably from about 10 to 60xc2x0 C. The absolute pressure is generally from about 0.1 to 2 MPa, most often from about 0.1 to 1.5 MPa.
The operating conditions of the distillation of stage b) for purification of the olefin comprise a pressure in the distillation column that is usually from about 0.1 to 1.5 MPa, preferably from about 0.2 to 1 MPa, identical or different to that of the washing. The distillation column usually comprises between 3 and 80 theoretical plates and most often between 10 and 50 theoretical plates.
The mixture that is obtained in stage a) contains at least one tertiary olefin, at least one alcohol and at least one residual ether. In the case where the washing is carried out before distillation, the washing stage makes it possible to collect a tertiary olefin-rich organic phase that contains olefin and at least a portion of the residual ether, and an alcohol-rich aqueous phase. The organic phase is then distilled. The tertiary olefin comes out at the top of the column, and the residual ether comes out at the bottom of the column.
The alcohol-rich aqueous phase is treated, preferably by distillation, to recycle water to purification stage b) and to recover the alcohol that is obtained in stage a). The synthesis of a tertiary alkyl ether, such as TAME or ETAE, can come from the reaction of an olefinic fraction with five carbon atoms that is produced by the catalytic cracking on a moving bed (FCC), steam cracking, dehydrogenation of isopentane or isomerization of branched olefins with five carbon atoms, with an alcohol. This alcohol can come in part from the alcohol-rich aqueous phase and is obtained in stage b) after the water is separated from the water of said phase.
In the case where the distillation is carried out before the washing, the distillation stage of the mixture that is formed in stage a) makes it possible to collect at the top of the column a tertiary olefin-rich organic phase that contains olefin and a portion of alcohol. At the bottom of the column, an ether-enriched organic phase that contains another portion of the alcohol is recovered. The organic phase is then washed with water to purify the olefin. The flows that contain alcohol can be, after optional purification treatments, sent at least in part to a unit for ether synthesis. The water that is obtained from the washing is advantageously recycled to purification stage b).
In the case of the synthesis of TAME, the alcohol that reacts with the olefinic fraction with five carbon atoms is the methanol. In the ETAE case, it is ethanol.
The tertiary olefin, preferably an isoamylene or a mixture of isoamylenes, that is obtained at the end of the second stage of the process according to the invention is dehydrogenated, in gas phase and in the presence of a catalyst, to form a diene, preferably isoprene.
Although the catalyst that is used in the dehydrogenation stage can be any type of catalyst that is able to carry out the dehydrogenation reaction, a catalyst that is resistant to the deposition of coke is preferably used to obtain improved selectivity and activity of the catalyst. The most used catalysts have a metal oxide base such as Co, Ni, Mo and W, and an alkaline metal oxide, generally on substrates of alumina, silica or silica-alumina. In a preferred form of the process according to the invention, a catalyst that consists of nickel oxide, molybdenum oxide and potassium oxide that are supported on an alumina-xcex3 is used.
The operating conditions for the oxidizing dehydrogenation stage comprise a (partial) hydrocarbon pressure that is usually from about 104 to 5.105 Pa, preferably between about 104 to 2.105 Pa. The temperature is generally between about 450 and 700xc2x0 C., preferably between about 600 and 680xc2x0 C. The feedstock weight/catalyst weight/hour (H.W.V.=hourly weight velocity) is usually between about 0.1 and 5 hxe2x88x921, preferably between about 0.5 and 2 hxe2x88x921.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding French application 98/11285, filed Sep. 9, 1998, are hereby incorporated by reference.
The following example illustrates the invention, without limiting its scope: